Add Serializable interface for comparators

It is not something important, just Java SDK recommendation: http://docs.oracle.com/javase/6/docs/api/java/util/Comparator.html

> Note: It is generally a good idea for comparators to also implement java.io.Serializable, as they may be used as ordering methods in serializable data structures (like TreeSet, TreeMap). In order for the data structure to serialize successfully, the comparator (if provided) must implement Serializable.